This invention relates to antenna input detecting circuits. More particularly, it relates to a ferrite rod antenna input coupling circuit for detuning the Q of the antenna to achieve both a broadband frequency response and an increase in the antenna's sensitivity when positioned near conductive parts, i.e. metal supports, printed circuit copper lands, chassis, etc.
It is known in the prior art that the Q of an antenna's input resonant tank circuit may be reduced (spoiled) by applying a portion of the detected antenna signal as negative feedback into the resonant tank circuit. By reducing the Q of the antenna resonant tank detecting circuit, a broadband frequency response is obtained. U.S. Pat. No. 2,787,704 illustrates the use of negative feedback to achieve a constant band-width input frequency response for a high-Q rod antenna. In this reference, the output from the rod antenna resonant tank circuit is buffered by a vacuum tube amplifier and a portion of the buffered signal applied through a feedback transformer into the resonant tank circuit. The secondary of the feedback transformer is connected in series with the tank inductor formed around the ferrite rod. Special tuning capacitors in the feedback circuit vary the feedback ratio of the feedback signal according to the frequency detected by the antenna to achieve a constant bandwidth regardless of the center frequency to which the antenna is tuned.
Some of the advantages of active Q-spoiling are also disclosed and discussed by this reference. That is, the sensitivity of the antenna is not diminished even though the Q of the antenna is reduced. (As understood by those skilled in the art, the antenna sensitivity or signal-to-noise ratio of its output signal, which is expressed in micro-volts per meter, refers to the amount of external magnetic field required to increase the antenna output signal by a factor 1.414 or 3db over the noise level when no external fields are present). This is true because the high input impedance of the active amplifier does not appear as a resistive load to the resonant tank circuit. Resistive loading of a resonant tank reduces the Q but increases the noise in the antenna output signal. Another advantage discussed by this reference relates to the broadband frequency response resulting from the detuning of a high-Q circuit. The negative feedback voltage in series with the resonant tank voltage reduces the Q of the tank to effectively open up the frequency band to detect more frequencies which occur near the resonant center frequency. This enables the antenna to effectively respond to several relatively separated frequencies.
One of the main advantages of a ferrite rod antenna is that it may be contained physically in quite a small volume. Therefore, in receivers which utilize rod antennas, it is inevitable that these antennas will be placed near other components, antenna's and metal parts. The resultant degradation of performance in the antenna's packaged configuration compared with that obtained when the rod antenna is isolated may amount to a having of the Q-factor of the antenna. In addition, the sensitivity of the antenna (signal-to-noise ratio of the detected output signal) is likewise reduced. When more than one rod antenna is placed in the same physical area, such as when two antennas are placed adjacent and orthogonal to one another, a problem of cross-talk between the antennas is created. This cross-talk between antennas produces undesirable signal responses in both antennas. In order to avoid this cross-talk problem, prior art receivers have resorted to elaborate configurations for the antennas. One such technique involves the use of four high-permeability ferromagnetic rods arranged in a square with the resonant tank coils for opposite rods interconnected to form a single effective rod antenna. For this technique, the physical configuration of the ferrite rods is critical in order for the antenna to maintain its desired frequency response and, at the same time, to minimize the cross-talk.
A long range navigation system which employs a plurality of transmitters transmitting on discrete frequencies, such as the Omega Navigational System, requires a signal receiver whose antenna is capable of detecting each of the very low frequency (VLF) signals that are transmitted. In such a receiver, two orthogonally positioned ferrite rod antennas are required in order to produce a detected signal regardless of the orientation of the receiver.
Accordingly, it would be desirable to provide a rod antenna input coupling circuit which enables the antenna to have a broadband frequency response while minimizing the cross-talk between adjacent rod antennas, and to have an increased sensitivity by reducing the antenna's internal losses due to the presence of nearby conductors.